Field of the Invention
The present invention relates to a liquid ejection device having mounted thereon a liquid ejection head configured to eject liquid.
Description of the Related Art
A liquid ejection device configured to record an image on a recording medium through ejection of liquid such as ink generally has mounted thereon a liquid ejection head configured to eject liquid. As a mechanism configured to eject liquid from the liquid ejection head, in many cases, there is used a mechanism configured to generate a pressure in a pressure chamber storing the liquid, to thereby eject, using the pressure, the liquid in the pressure chamber through an ejection orifice formed at one end of the pressure chamber. As methods of generating the pressure, there are given by, for example, reducing the capacity of the pressure chamber using a piezoelectric element, and by bubbling the liquid using a heating element to generate the pressure.
It is known that, in a liquid ejection head, presence of an air bubble in the pressure chamber considerably lowers droplet ejection performance. An air bubble is present in the pressure chamber due to various factors. For example, an air bubble is formed due to cavitation caused by pressure change in ejection or is brought into the pressure chamber from a supply flow path of the liquid. In order to remove such an air bubble from the pressure chamber, some methods are hitherto proposed.
For example, in Japanese Patent Application Laid-Open No. 2012-187862, there is disclosed a liquid ejection device in which a liquid circulating path including an upper tank, a liquid ejection head, a lower tank, and a circulation pump is formed. The upper tank is located above the liquid ejection head in a gravitational direction and can supply liquid to the liquid ejection head using a pressure head difference. The lower tank is located below the liquid ejection head in the gravitational direction and can recover the liquid from the liquid ejection head using a pressure head difference. The circulation pump is configured to return the liquid in the lower tank to the upper tank. With this configuration, the liquid ejection device disclosed in Japanese Patent Application Laid-Open No. 2012-187862 can record an image through ejection of the liquid from the liquid ejection head while the liquid is circulated along the circulating path described above. Through circulation of the liquid through the pressure chamber of the liquid ejection head in this way, not only an air bubble remaining in the pressure chamber can be removed together with the liquid but also thickening of the liquid in an ejection orifice can be suppressed.
Further, in the liquid ejection device disclosed in Japanese Patent Application Laid-Open No. 2012-187862, through driving of the circulation pump under a state in which an air release valve of the upper tank is closed to shut off a flow path between the liquid ejection head and the lower tank, pressurized liquid can be supplied to the liquid ejection head and can be discharged through the ejection orifice. Such pressurizing recovery operation enables droplet ejection performance to be satisfactorily maintained even in a liquid ejection head having a larger number of ejection orifices for attaining higher speed recording.
The liquid ejection device disclosed in Japanese Patent Application Laid-Open No. 2012-187862 is configured to pressurize the liquid via air in the upper tank in the pressurizing recovery operation described above. Therefore, at the end of the pressurizing recovery operation, compressed air in the upper tank expands until the pressure becomes equal to atmospheric pressure, and the expanded air causes the liquid to be kept discharged through the ejection orifices wastefully. Meanwhile, when, in order to suppress this problem, the air release valve of the upper tank is opened, the pressure of the compressed air abruptly becomes atmospheric pressure. The abrupt pressure reduction causes the ejection orifices to take in air, and as a result, the droplet ejection performance is lowered.